Add sample 66, adds assignment/print to expr'ns.

Next up:
* In the semi-big picture, I'm headed toward implementing js (start with
  jss) and a version of C.
* More short-term, I think it's time for some cleanup.

I think these are some next steps:
* Learn from the structure of samizdat.
* Separate working from broken samples.
* Build a much more comprehensive test system.
* Aggregate my TODO's; consider making them (or some of them) github
  issues.
* Consider other opportunities for code cleanup.